1. Field of the Invention
This invention relates to a heat-curable coating composition, and more specifically, to a heat-curable coating composition comprising an oil-free polyester, an alkyd resin, an acrylic resin and a crosslinking agent as binder components.
2. Description of the Prior Art
Heretofore, alkyd resin paints, acrylic resin paints and oil-free polyester paints have typically been used as heat-curable paints. These three types of paints have advantageous characteristics peculiar to the respective resins, but are not without defects.
The alkyd resin-type heat-curable paints have the advantage that they do not easily undergo popping (a pinhole-like paint film imperfection caused by solvent evaporation during baking or inclusion of bubbles during coating) and sagging (a trace of the flowing of a paint on a substrate after application which remains on the coated film after baking), and have excellent appearance (the superior gloss and high build of paint films), solvent resistance and chemical resistance. On the other hand, they have inferior weatherability (resulting in reduced gloss). Furthermore, a coated film of a deep color series obtained from the alkyd resin paint containing a large amount of a colored pigment excepting white pigments has poor resistance to waxing so that when such a coated film is waxed, the surface of the film tends to be scratched or develop hazy gloss, and the wax used in waxing is markedly colored. The waxing resistance is the property required of both a coated film immediately after coating and a coated film after outdoor exposure. It is a very important property because in an ordinary coating line, dirt and dust adhering to the coated surface are removed by wiping it with a wax, and the top coatings of automobile bodies are frequently waxed during use. It is difficult for the alkyd resin-type heat-curable paints to give a coated film having satisfactory waxing resistance, both before and after exposure, and a high level of weatherability (gloss retention).
The acrylic resin-type heat-curable paints have excellent weatherability, water resistance and chemical resistance, but have the defect that the appearance of the coated film (high build and smoothness), the mechanical properties (Erichsen test and impact resistance) of the coated film and the dispersibility of pigments are poor.
The oil-free polyester-type heat-curable paints have excellent weatherability, waxing resistance, mechanical properties, adhesion to a primer, and interlayer adhesion (adhesion between layers when a baked coated film is coated again with the same paint and the repainted film is baked), but have the defect that popping or sagging tends to occur during coating, and the dispersibility of a pigment is poor.
Attempts to obtain a heat-curable coating composition having only the advantageous characteristics of these resins by using two or more of these resins in combination have been unsuccessful because the alkyd resins, acrylic resins and oil-free polyester resins generally have poor compatibility with each other.